Solar fuel production has the potential to dramatically change the world's energy posture: from the prospecting and extraction of today, to renewable production using sunlight and atmospheric gases in the future. The issue in solar fuel production is not one of mere feasibility, as this can be accomplished via multiple pathways (e.g. thermochemical, electrochemical, even biological), but one of practical economic viability, expressed via metrics such as the levelized fuel cost, which is strongly tied to efficiency.
Solar concentration systems typically entail optics (mirrors or lenses) to focus a large area of sunlight, or solar thermal energy, onto a small area. The solar thermal energy may drive a heat engine, such as a steam turbine, which may be further coupled to an electrical power generator to convert a portion of the solar thermal energy into electricity. Solar concentration systems may also drive a thermochemical reaction to generate a fuel that chemically stores a portion of the solar thermal energy. Water splitting, gasification of coal, and reforming of methane are all under investigation as potential solar thermochemical fuel production techniques. Solar concentration systems may drive other important reactions on an industrial scale as well, such as CO2 reduction into CO, for example.
Many solar thermochemical reactions entail a redox cycle. Two-step thermochemical fuel production processes are a conceptually simple approach: a working material (oxide) is partially or fully reduced at a high temperature, then cooled and, in the case of water splitting, exposed to steam to be reoxidized and yield H2. The metal oxide is then reduced again to repeat the cycle. While identifying advantageous metal-oxides is currently a subject of research, thermodynamic considerations dictate the thermal reduction portion of the cycle generally requires a high temperature, typically between 1000-2000° C., depending on the reactive oxide chosen and other conditions in the system.
While two-step metal oxide cycles are promising, reduction to practice of these theoretically efficient processes has been challenging. Existing working materials, for example, have a low reversible oxygen capacity, yielding little H2 per mole oxide per cycle. The large energy requirement for heating the reactive material between cycle steps necessitates solid-solid heat recovery at high temperature. Maximizing the per-cycle yield drives operation towards very low thermal reduction pressures and very high thermal reduction temperatures. The former require large vacuum pumps or high-purity sweep gasses, and the latter lead to excessive aperture radiation losses and require the use of specialized materials.
Solar thermochemical reactors in which these materials are implemented can take many forms, affording more or less efficient fuel production, operability, scalability, etc. One conventional system utilizes a honeycomb substrate that is coated with the reactive oxide. The honeycomb substrate is alternately exposed to collected solar energy to heat the system and reduce the reactive oxide, and to a reactant gas, such as H2O in the case of water splitting, to generate fuel. Such a reactor is essentially a fixed bed operating in semi-batch mode, and as such, suffers temperature non-uniformities and low thermal efficiency because much of the solar energy is expended on heating non-reactive portions of the bed (e.g., honeycomb substrate) and is ultimately rejected from the system as waste heat, rather than utilized for fuel production. Also, with each redox cycle, the entire system undergoes extreme thermal cycling, leading to component fatigue and failure.
The broad question of reactor efficiency has been examined in detail by Siegel et al., (N. P. Siegel, J. E. Miller, I. Ermanoski, R. B. Diver, E. B. Stechel “Factors Affecting the Efficiency of Solar-Driven Metal Oxide Thermochemical Cycles” Ind. Eng. Chem. Res., 2013, 52 (9), 3276. DOI: 10.1021/ie400193q) arriving at the concept of the utilization coefficient as an indicator of achievable efficiency for a reactor-material combination in a two-step thermochemical cycle. In a recent analysis Miller et al. (J. E. Miller, A. H. McDaniel, M. D. Allendorf “Considerations in the Design of Materials for Solar-Driven Fuel Production Using Metal-Oxide Thermochemical Cycles” Adv. Energy Mater. 2014, 4, 1300469. DOI:10.1002/aenm.201300469) address efficiency from an even broader thermodynamic viewpoint, and establish a framework for materials design.
The current understanding is that the cost of solar collection is a dominant overall cost factor for solar fuels in general, and for specifically proposed thermochemistry-based system designs. It is therefore of substantial importance for further progress in this field to develop methods for determining the operating and design parameter space that maximize efficiency, given a reactor type and working material properties.
Methods which avoid many of the difficulties and efficiency limitations associated with existing reactor operation would advance the art of solar thermochemical fuel production.